A high Tc superconducting wire/tape is usually a composite consisting of a superconducting core and a metallic sheath. Many properties of the wire/tape depend on the metallic sheath materials. Additional layers over the metallic sheath can, however, modify the properties of the wire/tape, as described below in several examples.
(1) A superconducting device such as a motor, a transformer cable or a magnet requires additional insulating materials between the wires or between the adjacent turns of the winding in order to prevent short circuiting. According to EP 0 786 783 insulating layers have been applied between the superconducting layers formed by bare Bi-2223 tapes. High Tc wires having insulating surface layers could simplify the process of making superconducting devices and the volume of the devices could be reduced.
(2) The mechanical strength of a high Tc superconducting wire/tape depends on the sheath material. An Ag alloy sheathed tape is for instance much stronger than a tape sheathed with pure Ag. However, it is difficult to distinguish the Ag alloy from the pure Ag just by looking. The additional surface layer can be coloured or marked which enable to distinguish between different kinds of wire/tapes. It is common that a tape is annealed in a pan-cake or solenoid form. Asymmetry pre-stress could be built up during the annealing and therefore two sides of a high Tc tape could have different mechanical properties. It is therefore very important to be able to distinguish between the two different sides during a winding process, for instance by using different colours to distinguish between the two sides of the tape, one colour for the tensile stressed side and another colour for the compressed stressed side. As a result a degrading of the wire/tape could be omitted.
(3) The Ag or Ag alloy sheath is not completely gas tight or liquid tight. Long time exposure in air or long time in contact with liquid nitrogen could cause a degrading of the high Tc wire/tape. An additional layer could protect the tape from moisture, water, liquid nitrogen or other chemicals which could degrade the superconducting tape.
(4) The additional layer could change the surface friction of the wire/tape. A low friction is for instance needed for winding a superconducting cable.
Such additional layers could be applied by known techniques.
According to U.S. Pat. No. 4,927,985 an insulating layer is applied inside a conductor and the surface of the conductor is metallic. By this construction the insulating layer should be put in the conductor before the mechanical deformation and heat treatment. The materials suitable for the insulating layer are therefore restricted and organic materials cannot be used.
EP0044144 (U.S. Pat. No. 4,407,062) concerns a low Tc superconductor. Low Tc superconductors are totally different from high Tc superconducting materials. The low Tc materials mentioned in EP 0044144 is intermetallic. High Tc superconductors are ceramic. The method for producing the low Tc superconductor is therefore different from the method for producing high Tc superconductors.
Moreover the coating according to EP0044144 is applied before the final heat treatment
The coating materials are therefore for high temperature use and could for instance be composite of silicate, chalk and China clay.
Without elevated temperature heat treatment the coating is not stable. It can be simply removed by dipping in hot water and wiping (see EP0044144, page 10, lines 7-10).
From EP 04449316 A1 it is known to cover a superconducting wire with an organic coat so as to stabilize the superconducting wire against bending.